From a corrosion point of view, the presence of certain dissolved gases, even in small amounts, is undesirable in water systems which contact metal surfaces. For example, metal surfaces in contact with oxygen containing water can experience severe pitting in industrial water systems. Pitting is highly localized corrosion affecting only a small area of the total metal surface. This can be a serious problem, causing metal failure even though only a small amount of metal is lost and the overall corrosion rate is relatively low.
With respect to oxygen, the severity of attack will depend upon the concentration of dissolved oxygen in the water, pH and temperature. As water temperature increases, as for example in a water heating system such as a boiler, enough driving force is added to the corrosion reaction that small amounts of dissolved oxygen in the water can cause serious problems. Oxygen pitting is considered a most serious problem in boiler systems, even where only trace amounts of oxygen are present.
Deaeration is a widely used method for removing oxygen from an oxygen-containing aqueous medium. It is particularly useful for treating boiler feedwater and can be either mechanical or chemical.
While vacuum deaeration has proven to be a useful mechanical deaeration method for treating water distributing systems, boiler feedwater is treated using pressure deaeration steam as the purge gas. According to the pressure deaeration method for preparing boiler feedwater, the water is sprayed into a steam atmosphere and is heated to a temperature in which the solubility of oxygen in the water is low. Typically, greater than 99% of oxygen in the feedwater is released to the steam and is purged from the system by venting.
Mechanical deaeration is considered an important first step in removing dissolved oxygen from boiler feedwater. However, as already noted, as water temperature increases, even trace amounts of dissolved oxygen can cause serious problems. Accordingly, supplemental chemical deaeration is required.
For boilers operating below 1000 pounds per square inch (psi), catalyzed sodium sulfite is commonly used as an oxygen scavenger for the chemical deaeration of the feedwater. The oxygen/sulfite reaction can be effectively catalyzed by iron, copper, cobalt, nickel, and/or manganese. While the sodium sulfite oxygen scavenger is often used with success, this material does have recognized limitations. At boiler operating pressures of 900 to 1000 psi and above, increased dissolved solids from the sulfite/oxygen reaction product can become a significant problem. Also, at high pressures the sulfite decomposes in the boiler to form sulfur dioxide and hydrogen sulfide, both of which can cause corrosion in the return condensate system.
Hydrazine is also used as an oxygen scavenger. Hydrazine does not have the above noted high pressure limitation of sodium sulfite. For example, since the products of the hydrazine/oxygen reaction are water and nitrogen, no solids are added to the boiler water. Hydrazine as an oxygen scavenger does, however, have its own limitations. A major problem relates to the toxicity of hydrazine. Also, the hydrazine/oxygen reaction is very slow at low temperatures which may be encountered in some sections of a boiler system. The decomposition products of hydrazine are ammonia and nitrogen. The ammonia can be aggressive to copper or copper bearing metallurgies that are found in condensate systems.
In recent developments, the use of certain compounds such as dioxo-aromatic compounds or organically substituted derivatives thereof has become known. The group "dioxo-aromatic" consists of benzoquinone, napthoquinone, hydroquinone and catechol. The phrase "organically substituted derivatives thereof" includes any dioxo-aromatic compound having an organic substituent with a carbon atom attached directly to the aromatic ring. An example of such a derivative is 4-tertbutylcatechol. The use of quinones and diols as catalysts for the hydrazine/oxygen reaction in an aqueous medium is well known. See, for example, U.S. Pat. No. 3,551,349. U.S. Pat. No. 3,843,547 discloses the use of a combination of an aryl amine compound and a quinone compound as a catalyst for a hydrazine oxygen scavenger.